1. Field of the Invention
This invention relates to an optical low-pass filter for a color TV camera using a stripe filter for color separation and more particularly to an optical low-pass filter effectively usable for a color TV camera of the frequency division type.
2. Description of the Prior Art
A color TV camera of the type using a stripe color separating filter necessitates the use of a low-pass filter to cut off high spatial frequency. The prior art low-pass filters that have been proposed for this purpose include a filter using a double refraction plate, a filter using lenticular lenses, a filter using a phase grating, etc. Among these low-pass filters, a filter using a double refraction plate of crystal has been commercialized. The expense of crystal, has created a general desire for an inexpensive low-pass filter. To meet this desire, there have been made some proposals including, for example, stripe prisms disclosed in Japanese Laid-Open Patent Application No. SHO 55-38549.
In the accompanying drawings, FIG. 1 shows a conventionally known low-pass filter of the prism type. On one side of the low-pass filter 1, there are formed stripe prisms 2. Each of these prisms 2 has a vertex angle .delta.. Assuming that refractive index of the prism 2 is n, the angle of deviation .theta. of a ray of light passing through filter 1 is obtainable from the formula n.multidot.sin .delta.=sin (.delta.+.theta.). Since .delta. and .theta. are small, and therefore sin .delta..apprxeq..delta., sin .theta.=.theta. in radian, the single .theta. can be expressed by .theta.=(n-1).multidot..delta.. Assuming that the lens system in use is a telecentric system and that the distance between separated images at a distant point a is d, there is a relation expressed by d=2.theta..multidot.a. Applying this to a 2/3 inch image pick-up tube and assuming that the color carrier frequency is 3.58 MHz, the distance d required between separated image is d=0.02 mm. Then, assuming n=1.5 and a=8 mm, the vertex angle .delta. becomes .delta..apprxeq.8.6'. In cases where the lens system is not a telecentric system obtaining uniform separation over the whole image format, is achieved by positioning the filter within a parallel pencil of rays in front of the relay part of a zoom lens which corresponds to a pupil position of the whole optical system. The distance d between separated images can be expressed by d=2.theta.fR, where fR represents the focal length of the relay lens system. In this case with a focal length of 30 mm, the vertex angle .delta. of the prisms 2 must be .delta..apprxeq.2.3'. The value of the vertex angle .delta. thus obtained is almost equal to the finishing precision, therefore, a manufacturing problem is presented.